1. Field of the Invention
The present invention relates to a substrate support structure for clamping a substrate on a surface thereof.
2. Description of the Related Art
Clamping of a substrate, for example a wafer, onto a surface of a substrate support structure, for example a wafer table, is well known in the semiconductor industry, and in particular in lithography systems. In such lithography systems, the clamped substrate is patterned by exposure to incident photons or charged particles such as ions or electrons. The clamping permits the realization of high precision patterning of a target portion of the substrate surface.
One method of clamping is by sucking away air between the substrate and the substrate support structure, i.e. by creating a vacuum in between them. However, if the clamped substrate is to be processed in a vacuum environment, this method will not be effective. Various other solutions exist for clamping a substrate in a vacuum environment, for example by means of electromechanical clamping. However, this solution is not suitable for use with charged particle lithography due to the undesirable influence of the electric fields used for clamping on the beams of charged particles.
Another method of clamping which avoids the above problems is by using a liquid layer arranged to induce a capillary force such that the substrate is clamped on a surface of a substrate support structure. Adhesion of the liquid to the surfaces of the substrate on the one hand and the substrate support structure on the other hand creates a circumferentially extending liquid surface, concavely extending between the two surfaces. The so-formed concave liquid surface tends to maintain its shape, even if forces are applied to remove the substrate from the substrate support structure surface. The liquid layer may further serve the purpose of enhancing the thermal contact between the substrate and the substrate support structure surface to enable the substrate to withstand higher heat loads without being subject to excessive contraction or expansion.
However, clamping using a liquid clamping layer suffers from several drawbacks. Evaporation of the clamping liquid layer results in deterioration of the clamping force over time, which limits the useful lifetime of the clamp. Leakage of vapor from the liquid layer is also a problem for many applications, such as a lithography process in which the clamped substrate is introduced into a vacuum environment and water molecules from vapor leaking into the vacuum chamber are contaminants detrimental to the lithography process. Condensation of vapor from the clamping liquid may also pose a problem, reducing the lifetime of the clamp.